Evidence of non-LTE effects in mesospheric water vapor as determined by infrared spectral emission measurements taken from the space shuttle is reported. A cryogenic Michelson interferometer in the CIRRIS-1A shuttle payload yielded high quality atmospheric infrared spectra. These measurements demonstrate the enhanced daytime emissions of H2O (020→010) which are the result of non-LTE processes and in agreement with non-LTE models. The radiance ratios of H2O (010→000) and (020→010) Q(1) transitions during daytime are compared with non-LTE model calculations to assess the vibration-to-vibration exchange rate between H2O and O2 in the mesosphere. An exchange rate of 1.2¿10-12 cm3 s-1 is derived. ¿ 1999 American Geophysical Union